Because exhaust gases discharged from diesel engines, etc. contain soot-based particulate matter (PM) and nitrogen oxides (NOx), which are likely to adversely affect humans and environment when discharged into the air, the development of technologies for reducing PM and NOx have been investigated. Thus, exhaust members of diesel engines, etc. are provided with PM-capturing filters and NOx-removing catalyst carriers.
One example of ceramic honeycomb filters 10 for capturing PM in the exhaust gases of automobiles is shown in FIGS. 1(a) and 1(b). The ceramic honeycomb filter 10 comprises a ceramic honeycomb structure 11 comprising porous cell walls 14 defining a large number of outlet-side-sealed flow paths 15a and inlet-side-sealed flow paths 15b and a peripheral wall 11a, and upstream-side plugs 13a and downstream-side plugs 13b sealing the exhaust-gas-inlet-side end surface 12a and exhaust-gas-outlet-side end surface 12b of the outlet-side-sealed flow paths 15a and inlet-side-sealed flow paths 15a alternately in a checkerboard pattern. An exhaust gas (shown by arrows of dotted lines) flows into the paths 15b open at the end surface 12a, passes through the cell walls 14, and flows out of the paths 15a open at the end surface 12b. While passing through the cell walls 14, PM in the exhaust gas is captured by the cell walls 14, so that the exhaust gas is cleaned.
The ceramic honeycomb filter 10 is contained in a metal vessel (not shown) with the peripheral wall 11a of the ceramic honeycomb structure 11 held by members of metal mesh or ceramic mat, etc. to prevent it from moving during operation. As a catalyst carrier for removing NOx, a ceramic honeycomb structure 11 without upstream-side plugs 13a and downstream-side plugs 13b is used.
The ceramic honeycomb filter 10 as shown in FIGS. 1(a) and 1(b) is produced by the following steps: (a) a step of blending ceramic (for example, cordierite) materials, a binder, a pore-forming material, etc. to prepare a moldable material, (b) a step of extruding this moldable material, for example, by a screw-type extruder to form a honeycomb-structured green body, which is cut to a larger length than a target length, taking into consideration deformation in a drying or sintering step, (c) a step of drying and sintering the green body to form a cordierite ceramic honeycomb structure, (d) a step of machining end surfaces 12a, 12b of this ceramic honeycomb structure by a grinding tool such as a diamond cutter, a diamond saw, etc. to produce a honeycomb structure 11 having a predetermined length, and (e) a step of plugging the paths 15a, 15b at both end surfaces 12a, 12b of the honeycomb structure 11 with a plugging material in a checkerboard pattern, and drying and sintering the plugging material to obtain a honeycomb filter 10 having upstream-side plugs 13a and downstream-side plugs 13b. 
The ceramic honeycomb filter is required to have low pressure loss. To provide the ceramic honeycomb structure with desired porosity, investigation has been conducted on the control of the particle sizes of ceramic materials, the use of organic pore-forming materials and the control of their amounts, etc. However, organic pore-forming materials and/or organic binders used for the ceramic honeycomb structure are burned in a sintering step, generating thermal stress affecting the honeycomb structure, thereby causing cracking.
To solve the above problems, JP 2004-142978 A discloses a method for producing a porous honeycomb structure by blending ceramic or metal aggregate particles, water, an organic binder, a pore-forming material, and colloid particles to prepare a moldable material, molding and drying the moldable material to a honeycomb-shaped green body, calcining the honeycomb green body, and sintering the resultant calcined body. JP 2004-142978 A describes that because the colloid particles are hardened by a dehydration condensation reaction, etc. at relatively low temperatures, they act as a reinforcing agent after burning off the organic binder, preventing the mechanical strength of the green body and the porous honeycomb structure from decreasing, so that the porous honeycomb structure is prevented from being cracked even when the rapid temperature elevation of a green body being sintered generates large thermal stress, due to the combustion of large amounts of the binder and the pore-forming material.
However, in the production method of a ceramic honeycomb structure described in JP 2004-142978 A, a large amount of colloid particles should be added such that they can act as a reinforcing agent, and excess colloid particles likely make it difficult to obtain a target ceramic composition. In addition, some types of organic pore-forming materials may be burned at lower temperatures than the dehydration condensation reaction temperature of colloid particles, making it difficult to completely avoid cracking due to the combustion of the organic pore-forming material. Particularly in the case of large honeycomb structures of 150 mm or more in outer diameter and 150 mm or more in length, to which a large amount of an organic pore-forming material is added to have cell walls having as high porosity as 40% or more, the reinforcing effect of the colloid particles is not sufficiently obtained.
JP 2010-001184 A discloses a method for sintering aluminum titanate using a thermoplastic resin having a thermal decomposition start temperature of 400° C. or lower as a pore-forming material, in which a low-oxygen atmosphere having an oxygen concentration of 2% or less is kept from a start temperature to an oxygen-introducing temperature of 1100° C. or lower in a step of heating a honeycomb green body to a sintering temperature, and oxygen is introduced at temperatures equal to or higher than the oxygen-introducing temperature, such that the oxygen concentration is higher than 2%. JP 2010-001184 A describes that when temperature elevation starts in the low-oxygen atmosphere, only an endothermic reaction of decomposing the resin occurs, providing no thermal stress due to heat generation, so that high-quality exhaust gas filters free from sintering cracking can be formed at high productivity without taking much time for sintering.
In the production method of a ceramic honeycomb structure described in JP 2010-001184 A, however, it is difficult to completely suppress cracking due to the combustion of the organic pore-forming material, for example, when a large amount of an organic pore-forming material is added to obtain cell walls having as high porosity as 40% or more in a large honeycomb structure of 150 mm or more in outer diameter and 150 mm or more in length. Because an additional apparatus for sintering in a low-oxygen atmosphere is used, large investment for the production is needed. In addition, the complete decomposition of some types of organic pore-forming materials in a low-oxygen atmosphere likely takes time to some extent, needing a long sintering time.
JP 08-323123 A discloses a method for producing an exhaust gas filter comprising molding a moldable ceramic material containing an ethylenic resin softened at 120° C. or lower as a pore-forming material, drying the resultant green body around a temperature at which the pore-forming material is softened, and sintering it. It describes that the softening of the pore-forming material during drying makes a high-fluidity pore-forming material exist more on the cell wall surfaces than in the cell walls, thereby providing larger pores on the cell wall surfaces than those in the cell walls after sintering, and that therefore, even the pore-forming material added in a small amount can provide the exhaust gas filter with practical pressure loss performance.
However, the production method of a ceramic honeycomb structure described in JP 08-323123 A is to control the diameters of pores on the cell wall surfaces, and most of the pore-forming material remains at the time of sintering though softened during drying. Accordingly, it fails to solve the problem of cracking by thermal stress generated by the combustion of the pore-forming material.
Particularly when a large amount of an organic pore-forming material is added to obtain cell walls having as high porosity as 40% or more in large honeycomb structures of 150 mm or more in outer diameter and 150 mm or more in length, it is not easy to suppress cracking due to the combustion of the organic pore-forming material, so that new improvements are desired.